1. Field of the Invention
The present invention relates to an optical fiber amplifier. More specifically, the present invention pertains to an optical fiber amplifier having a light surge preventing function capable of preventing light surge, while when light output power of input signal light is decreased, light output power of pumping light derived from a pumping light source and entered into an amplifying optical fiber is suppressed.
2. D escription of the Related Art
One conventional optical fiber amplifier is arranged by an optical fiber for amplifying signal light, a pumping light source for outputting pumping light, and a light multiplexing device for inputting the pumping light to the optical fiber. Furthermore, in the conventional optical fiber amplifier, a light branching device and a light receiving device are arranged at the post stage of the optical fiber, and these light branching device and light receiving device monitor the amplified signal light. A light output level of the amplified signal light is detected by this light receiving device so as to control the pumping light output from the pumping light source. Normally, optical isolators, or optoelectronic isolators are arranged before/after the optical fiber along the signal light path in order to avoid adverse influences such as reflection light.
The signal light externally entered into the optical fiber is amplified by the wavelength-multiplexer based on the pumping light entered into this optical fiber, and then the amplified signal light is outputted outside from the wavelength-multiplexer. The above-explained arrangement of the conventional optical fiber amplifier and the basic operation thereof are described in, for instance, Japanese book "LIGHT AMPLIFIERS AND THEIR APPLICATIONS" written by Ishio et al., published by Ohm sha, 1992, page 111.
As the amplifying optical fiber used in the above-described optical fiber amplifier, the rare earth doped optical fiber into which the rare earth element has been doped is employed.
However, in the optical fiber amplifier with employment of such a rare earth doped optical fiber having the function to amplify the input light thereof, in which the rare earth element has been doped into the optical fiber, the lower the level of the entered signal light becomes, the higher the inverted population becomes. As a consequence, very large energy is stored under such a state that no external signal is inputted. Accordingly, when the signal light is suddenly inputted from such a condition that no signal light is entered into the optical fiber amplifier with employment of the rare earth doped optical fiber, the induced emission will occur due to the stored large energy, the light surge with the high power will be produced since this induced emission occurs until the inverted population could reach the balance condition. Then, this high power light surge is outputted from the optical fiber amplifier. Since the light surge is produced in the rare earth doped optical fiber, this light surge would give the adverse influences to the optical components arranged at the post stage of this optical fiber.